The first of two groups of psychophysical experiments proposed here will investigate the impact of receptor spacing in the retinal mosaic on visual acuity, with particular emphasis on spatial resolution for targets seen only by short wavelength-sensitive (B) cones. B cones are sparsely represented in the retinal mosaic, allowing their distribution to be mapped with psychophysical techniques. Two techniques will be developed to assess B cone spacing in the intact human eye. B cone acuity will be measured with laser interference fringes which bypass the attenuating effects of the eye's dioptics. This will provide a unique opportunity to compare cone spacing and visual acuity in the same retinal locations of the same observer. Other investigators have drawn a relationship between cone spacing and resolution for luminance gratings by invoking the sampling theorem, that states that the highest spatial frequency that can be unambiguously reconstructed has a period twice the distance between receptors. Measures of B cone spacing, in conjunction with measures of chromatic spatial resolution for gratings seen by B cones can be compared with the predictions of the sampling theorem, including the prediction that aliasing or beating effects might be demonstrable. The second group of experiments provide two psychophysical tests of the principle of univariance, applied to rod vision, that are motivated by electrophysiological finding that the time course of singals from rods may depend not just on the number of quanta caught by the receptor, but also on the location of stimulation along the outer segment. The scotopic temporal frequency response will be measured to see if it is dependent on wavelength or direction of incidence of light, since such a dependence implies a failure of univariance.